JP 2007-295758A discloses a surge protection device for protecting a semiconductor element that is connected to a power supply to operate, from a negative surge current. This surge protection device includes a reverse current prevention diode that prevents a current from flowing backward to a semiconductor element, and a voltage limiting circuit that limits a reverse voltage that is applied to the semiconductor element, to be no greater than the withstand voltage of the semiconductor element. The surge protection device has a configuration in which, if a negative surge current intrudes into the power supply while the semiconductor element is being supplied with power from the power supply, the direction in which the voltage is applied to the reverse current prevention diode is switched from a forward direction to a reverse direction.
If power supply to a load is controlled by switching between switching elements, there is the risk of erroneous blocking occurring in a switching element due to a potential rise that is caused by a current that is applied to the load. The following describes this problem with reference to the simplified diagram shown in FIG. 4, for example.
FIG. 4 illustrates an on-board system Sy that includes a load driving device D that controls power supply from a power source unit Ba to a load Pa by switching a switch unit IP. FIG. 5 shows changes in the potential of each part of the load driving device D. In this load driving device D, when a load driving request signal is input to a microcomputer M in response to a switching operation of an external switch, the microcomputer M outputs a high level signal to a signal line La. The high level signal output from the microcomputer M is input to the switch unit IP as a signal having a predetermined potential, via a driving circuit Ca that is constituted by a resistor R1 and a resistor R2. A semiconductor switching element SW provided in the switch unit IP performs an ON operation when the potential of the signal (input potential) input to the switch unit IP via the driving circuit Ca is greater than the potential of a conducting path (P-GND) that is interposed between the semiconductor switching element SW and the load Pa, by a predetermined value or more. In the example shown in FIG. 5, the switch unit IP is performing the ON operation at “LOAD-ON” because the input potential to the switch unit IP and the potential of the conducting path (P-GND) is greater than an ON-threshold value.
In the on-board system Sy shown in FIG. 4, upon a current flowing from the power source unit Ba into the load Pa due to the ON operation of the switch unit IP (i.e. the ON operation of the semiconductor switching element SW), the potential of the conducting path (P-GND) interposed between the semiconductor switching element SW and the load Pa rises after, for example, “LOAD-ON” shown in FIG. 5. On the other hand, the output voltage that is applied from the microcomputer M to the signal line La (e.g. approximately 5 V) is lower than the output voltage from the power source unit Ba, and the reference of this output voltage is the ground potential of the microcomputer M (the potential of S-GND that is maintained at 0 V, independently of ON and OFF of the switch unit IP). Due to such a configuration, if the potential of the conducting path (P-GND) rises due to a current that is applied in response to the ON operation of the semiconductor switching element SW after, for example, “LOAD-ON” shown in FIG. 5, there is the risk of the difference between the input potential supplied from the driving circuit Ca to the switch unit IP and the potential of the conducting path (P-GND) becoming lower than a value (the ON-threshold value) that is required to cause the switch unit IP to perform the ON operation. If such a situation occurs, erroneous blocking occurs, which is a phenomenon in which, for example, the switch unit IP performs an OFF operation despite a driving signal being continuously output from the microcomputer M to the signal line La. In the example shown in FIG. 5, such erroneous blocking occurs at “UNINTENDED LOAD-OFF”. After the erroneous blocking occurs, the difference between the input potential to the switch unit IP and the potential of the conducting path (P-GND) increases again, and therefore the switch unit IP performs the ON operation again. Consequently, such unintended ON and OFF operations are repeatedly performed.